#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>

int main()
{
   int arr[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 };
   int n = 0;
   scanf("%d", &n);
   int sz = sizeof(arr) / sizeof(arr[0]);
   int right = sz - 1;
   int left = 0;
   while (left <= right)
   {
       int mid = (right + left) / 2;
       if (arr[mid] > n)
       {
           right = mid - 1;
       }
       else if (arr[mid] < n)
       {
           left = mid + 1;
       }
       else if (arr[mid] == n)
       {
           printf("找到了下标是：%d\n", mid);
           break;
       }
   }
   if (left > right)
   {
       printf("找不到了\n");
   }
   return 0;
}
